The present invention is directed toward a method and apparatus for demodulating high speed Time Division Multiplexed signals and, more particularly, toward a method and apparatus for demodulating high speed Time Division Multiplexed signals by the use of an address list precursor.
With the opening of higher radio frequency bands, such as a Ka band, to wireless communication systems (both terrestrial and satellite), the allocated bandwidth to individual systems has increased dramatically. For example, an award has been made of 1 GHz of spectrum, consisting of 500 MHz uplink and 500 MHz downlink, for broadband multi-media satellite services in the U.S. utilizing low earth orbiting satellites.
In a wireless communication system, where data packet switching is employed and Time Division Multiplexing is the selected mode of downlink access, it is advantageous to use a small number of broadband carriers in the downlink, as opposed to a large number of narrow band carriers. This means that the data packets destined for a multiplicity of earthbound user terminals are time multiplexed into a single broadband, high data rate carrier. However, the data demodulation rate at an individual user terminal may be desired to be much smaller than the carrier data rate, also referred to as the bearer data rate, to reduce demodulator complexity and cost. For example, an exemplary bearer data rate may be 500 Mbits/sec, where an exemplary demodulation rate at an individual user terminal may be 2 Mbits/sec.
A prior art method of demodulating Time Division Multiplexed (TDM) data packets consists of demodulating all data packets in the downlink carrier in real time. Accordingly, the user terminal demodulation rate has to be at least as great as the bearer modulation rate in order for complete demodulation to occur. The Inmarsat-C mobile satellite data transmission system is an example of such a system where the user terminal demodulation rate equals the bearer data, or modulation, rate. However, the bearer data rate in the Inmarsat-C is considerably lower (600 bits/sec) than applications of interest when transmitting in the higher radio frequency bands. Using the above example where a 500 Mbit/sec bearer data rate is implemented, xe2x80x9creal timexe2x80x9d demodulation, as provided by the Inmarsat-C, would mean that the user terminal would be required to have a 500 Mbit/sec demodulator. This sets the complexity and cost of the demodulator at the user terminal at a much higher level than if, for example, a 2 Mbit/sec demodulator were provided at the user terminal.
An alternative prior art method consists of utilizing a Time Division Multiple Access (TDMA) link, wherein the time axis is divided into frames, which are further subdivided into a multiplicity of slots. A plurality of receiver terminals are assigned to the time slots, one receiver to a slot, for a finite length of time on a demand basis. In this manner, each receiver needs to demodulate at only approximately the rate of R/M, where R is the bearer data rate and M is the number of slots per frame. This xe2x80x9cless than bearer data ratexe2x80x9d demodulation is possible because each receiver has exact knowledge of when its designated data packet will arrive, and therefore need not demodulate other segments of the received carrier. Although this technique allows for a reduction of the demodulation rate by a factor equal to the number of slots in a frame, typically by a factor of 7, the resulting required demodulation rate, typically 70 Mbits/sec, requires a demodulator still much greater in cost, complexity of implementation and power dissipation than a 2 Mbit/sec data rate demodulator.
The present invention is directed toward overcoming one or more of the above-mentioned problems.
A method of demodulating a communication signal transmitted at a first rate is provided, wherein the communication signal includes a plurality of addresses and corresponding data packets. The demodulating method includes the steps of receiving the communication signal at a user terminal, identifying which of the plurality of data packets are destined for the receiving user terminal, and demodulating, at a second rate, only the data packets identified as being destined for the receiving user terminal.
In one aspect, the step of identifying which of the plurality of data packets are destined for the receiving user terminal includes the steps of demodulating the plurality of addresses, and detecting which of the plurality of addresses matches the receiving user terminal address.
In another aspect, the step of receiving the communication signal at the user terminal includes the steps of writing the received communication signal to a memory in the user terminal, and reading the communication signal from the memory for demodulation.
In another aspect, the second rate is less than the first rate.
In another aspect, the communication signal includes a TDM signal.
In another aspect, the communication signal is transmitted from an Earth-orbiting satellite.
In another aspect, the plurality of addresses and corresponding data packets are arranged by grouping the plurality of addresses together followed by the plurality of data packets.
In another aspect, the plurality of addresses and corresponding data packets have a one-to-one correspondence.
In another aspect, the plurality of addresses and corresponding data packets are arranged in an interleaved manner, wherein each address is immediately followed by its corresponding data packet.
In another aspect, the first rate is approximately 500 Mbits/sec.
In another aspect, the second rate is approximately 2 Mbits/sec.
In another aspect, the communication signal further includes a preamble having a continuous wave segment followed by a synchronization word, wherein the demodulating method further includes the steps of detecting a presence of energy in the continuous wave segment of the preamble, the detection of energy indicating receipt of the communication signal at the user terminal, and filtering the synchronization word with a matched filter at the user terminal to confirm the detection of energy in the continuous wave segment.
In yet another aspect, the continuous wave segment includes an unmodulated, pure carrier wave.
In still another aspect, the synchronization word includes a bit pattern having a zero correlation with a shift in itself.
An apparatus is also provided for demodulating a communication signal received at a user terminal at a first rate, wherein the communication signal includes a plurality of addresses and corresponding data packets. The apparatus includes a digital memory receiving and storing the sampled and digitized communication signal at the first rate, an address list preprocessor receiving only the signal samples corresponding to the plurality of addresses from the memory, demodulating the plurality of addresses, and detecting which of the plurality of addresses matches the receiving user terminal address, and a demodulator receiving only the signal samples corresponding to the data packets from the memory whose corresponding addresses match the receiving user terminal address, and demodulating the received data packets at a second rate less than the first rate.
In one aspect, the apparatus further includes a memory manager controlling operation of the memory.
In another aspect, the communication signal is transmitted from an Earth-orbiting satellite.
In another aspect, the communication signal includes a preamble having a continuous wave segment followed by a synchronization word, the apparatus including means for detecting the presence of energy in the continuous wave segment of the preamble, the detection of energy indicating receipt of the communication signal at the user terminal, and means for detecting the synchronization word to confirm the detection of energy in the continuous wave segment.
In yet another aspect, the apparatus includes means for detecting the synchronization word in real time by a real time matched filter.
In still another aspect, the apparatus includes means for detecting the synchronization word in non-real time by a non-real time matched filter operating on stored samples of the synchronization word.
An object of the present invention is to provide a method of demodulation for high speed TDM packet data while reducing the complexity and the power dissipation of the demodulator at the user terminal so as to approach those of a demodulator that continuously demodulates only its own data.
Another object of the present invention is to provide a demodulation system capable of demodulating high speed TDM packet data while maintaining a reduction in the complexity and power dissipation of the demodulator at the user terminal so as to approach those of a demodulator that continuously demodulates only its own data, and not data destined for another user terminal.
Other aspects, objects and advantages can be obtained from a study of the application, the drawings, and the appended claims.